Furnace assembly for firing dental products

ABSTRACT

A furnace assembly for use in firing or fusing ceramic dental products is disclosed. The furnace includes an electrically heated muffle which may be operated at a desired vacuum condition during a firing or fusing cycle of operation. The furnace is heated at a controlled incremental rate of temperature rise from a predetermined minimum to a predetermined maximum temperature over a predetermined time period.

United States Patent 1 1 Jensen et al.

[4 1 Sept. 30, 1975 1 FURNACE ASSEMBLY FOR FIRING DENTAL PRODUCTS [75] Inventors: Robert A. Jensen, Hazel Crest; John A. Tesk, Woodridge; Daniel Odulio, Chicago, all of 111.

52 US. Cl 219/390; 13/31 [51] Int. Cl. F27B 5/14; F27D 11/02 [58] Field of Search 219/390, 407, 536; 13/22, 13/25, 31

[56] References Cited UNITED STATES PATENTS 1,724,956 8/1929 Monchton 219/390 1,930,836 10/1933 DAmico 219/390 2,398,874 4/1946 Weyhing... 219/390 3,109,911 11/1963 Kremer....... 219/390 3,541,293 11/1970 MacDonald et al. 219/390 Primary E.\'mninerVol0dymyr Y. Maycwsky Attorney, Agent, or FirmPennie & Edmonds [57] ABSTRACT A furnace assembly for use in firing or fusing ceramic dental products is disclosed. The furnace includes an electrically heated muffle which may be operated at a desired vacuum condition during a firing or fusing cycle of operation. The furnace is heated at a controlled ineremental rate of temperature rise from a predetermined minimum to a predetermined maximum temperature over a predetermined time period.

3 Claims, 7 Drawing Figures US. Patent Sept. 30,1975 Sheet 1 of 4 3,909,590

US. Patent Sept. 30,1975 Sheet 2 of4 3,909,590

FIG.2.

US. Patent Sept. 30,1975 Sheet 3 0M 3,909,590

FIG. 4.

US. Patent Sept. 30,1975 Sheet4 of4 3,909,590

FIG].

}To I50 }"ON-OFF"CURREN*T CONTROL FIG.6.

FURNACE ASSEMBLY FIRING DENTAL PRODUCTS BAcKoRouNo oF THE INVENTION The art of fabrication of ceramic dental products such as artificial teeth is known. Thus, while artificial teeth may be readily available there is a need to fabricate oral restorative devices which are custom items satisfying specific requirements of individual patients. These include porcelain facings, veneers, porcelain bridges, porcelain inlays, ceramic jacket crowns, and a host of other porcelain dental products. Generally, these products are fabricated by a technique including steps of application of a slurry to a base member or substrate; followed by heating of the applied coat to predry the same over a time period at below firing temperature; and, finally subjecting the product to a fusing or firing heating cycle. Successive steps of like nature may be undertaken if additional slurry layers are condensed on or applied to the substrate for purposes of fabricating finished products of required shape, shade or color, and dimension for particular use in a restorative device.

The predrying operation conveniently may be carried out in a separate furnace from that in which a firing of the various layers is carried out.

At the present time, dental products generally are fired in a vacuum atmosphere. It has been considered that the vacuum atmosphere provides improved structural results through the increase in density of the applied layers. It has also been found and considered that the vacuum atmosphere results in aesthetic improvements, particularly in the transluceney of the finished product. For this purpose, the product will be subjected to the vacuum atmosphere upon the commencement of .the firing cycle and before any possible sealing because of vitrification of the surface layer.

Furnaces to effect the ultimate evacuation and firing or fusing of the various layers comprising composite ceramic dental products of the above described type are formed by shells of relatively small capacity to permit rapid evacuation and heating of the interior. The shell will provide an access opening to the interior to facilitate application of a suction force and create a negative pressure for the desired creation of aesthetic effect. The shell interior will be capable of being heated to carry out the firing or fusing of product.

Heating of the shell interior has been accomplished in the past by application of electrical resistance rods or coils of resistance wire applied to the exterior surface of the shell for transmission of heat by conduction through the wall to the interior compartment. A base wound technique for the same purpose has also been used in the industry. Many forms of control of current input to the windings for the control of heat to the interior compartment have been employed.

To this end, a control of current input has been provided by a variable transformer, either manual or motor driven, and solid state current regulators. This type of control, while widely used, is considered to introduce problems to the procedure of firing or fusing a dental prosthesis. Thus, furnaces constructed with variable transformers, either manual or motor driven and solid state current regulators each attempt to control BTU input to the heated zone by regulating current input. As an example, itmay be determined that a current input of 8 /2 amps should be sufficient to cause the furnace to rise an average of 86F. per minute over a five minute period. However, the temperature rise; may

unot be linear overthetime period. In this connection,

for the first minute the temperature may rise F. During the ensuing time interval, the rate of rise may decrease so that the rise during the last minute is only 50F, This condition may occur during the first firing or fusing cycle. During a subsequent firing or fusing cycle, which may be spaced in time from the first, the temperature with the same current input may rise to a maximum desired over a shorter time period.

The variation in temperature rise is dependent upon how many BTUs of heat are stored in the firing chamber wallat any given time. The operator never knows accurately what current input to use to carry out the required rise in temperature of the chamber over the time period. Therefore, consistent quality may be difficult to realize or never realized. To this end, the shades of color may be inconsistent and the transluceney and amount of glaze of porcelain can vary, causing extra labor and/or inconsistent quality.

BRIEF DESCRIPTION OF THE INVENTION To overcome the above problem, a novel approach in controlling the firing of dental prostheses is carried out. In one aspect of the present invention, the interior temperature of a furnace chamber is controlled through programming the temperature by means of the firing cycle. To this end, a straight line firing cycle, within approximately +5F., may be programmed by setting a time proportioning switch to a desired rate of climb.

As a further aspect, the present invention is directed to a furnace operating under a controlled rate of temperature climb and subjected both before and during the firing period to a negative pressure.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof'that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. Those skilled in the art will appreciate that the conception upon whichthis disclosure is based may readily be utilized as a basis for the designing of other structures for carrying out the several purposes of the invention. It is important, therefore, that the claims be regarded as including such equivalent construction as do not depart from the spirit and scope of the invention.

DESCRIPTION OF THE DRAWINGS The accompanying drawings form a part of the present application. By these'drawings which illustrate a preferred form of the invention,

FIG. 1 is an illustration in perspective of the housing structure for the vacuum furnace apparatus of the present invention;

FIG. 2 is a schematic presentation of the vacuum chamber and furnace muffle therein;

FIG. 3 is a side elevational view of the vacuum furnace apparatus of FIG. 1 having a portion of the side wall broken away thereby to illustrate internal vacuum chamber structure;

FIG. 4 is a detailed view of the FIG. 3 shown with the furnace muffle in vertical section and a portion of the the vacuum module; and

FIG. 7- is a schematic electromechanical diagrarnillustrating the operation of 'thetc'mperature program,- mer and controller.

DESCRIPTION OF- THE PREFERRED EMBODIMENT The porcelain vacuum firing furnace having programmed temperature capability is represented generally by the numeral 10 in FIG. 1. The apparatus includes a plurality of operative components including a vacuum furnace assembly 12, a vacuum module 14, a current controller 16, and an automatic temperature controller 18. As illustrated, the vacuum module is received within the furnace assembly; whereas, the current and temperature controllers are housed within a base cabinet assembly 20.

The furnace of the present invention is designed to fire or fuse porcelain under vacuum conditions and during fan accurately programmed temperature cycle. More particularly, the porcelain coated crown or bridge isintroduced into the furnace whose internal temperature is at approximately 1400F. Preferably,

the porcelain crown or bridge will have been preheated during a time period which may vary between approxirnately four to about twenty minutes dependent upon, .for example, the thickness of thecondensed porcelain layer and its moisture content. A temperature of, for

4 The vacuum furnace, assembly provides an opening in the front wall for communication with the interior. The opened end of the vacuum chamber is mounted by the front wall in surrounding relation to the opening. Suitable sealing means in the region of the connection are necessary to maintain the negative pressure to be developed within the chamber'prior to and maintained during the firing or fusing procedure'A door 48 is received on the housing to close communication to the vacuum chamber. As may be apparent, the door will be insulated and both received and secured in sealed fashion on the housing. Any particular sealing structure for .this purpose may be utilized,such as a sealing gasket of suitable silicone rubber or the like disposed between the door and front wall.

A vacuum module 50 (see also FIG. 5) is supported within the furnace assembly. In the illustrated embodiment the module is received within housing beneath the vacuum chamber. The module may be slidably received and supported by a track frame (not shown). The vacuum module should be capable of achieving operating characteristics such as, among others, a vacuum force of 27 in. Hg, a vacuum flow of 3.2 scfm, and air consumption of 4.6 scfm, as discussed herein.

The module includes a pump or vacuum transducer 52 providing an air supply inlet 54, an exhaust outlet 56, and a vacuum port 58. The inlet is coupled to a source of compressed air by means of an elbow 60, connector 62, connector coupling 64 and hose assembly 66. In operation, compressed air from a source is passed through a small orifice at the transducer inlet and exhausted into a venturi section within the transducer. The throat of the 8. This procedure, as is wel known, results in an increase in the air velocity and the creation of a negative pressure at the throat. Adjustment of the air supply pressure provides regulation of the vacuum force attained. To thisend, the module on a front face carries a dial 68 which is movable-within limits to control a vacuum regulator 70 in the form of d a switch 72 (see FIG. 6). The switch, in turn, controls tained. The furnace chamber'will have been evacuated and a fixed pressure of approximately 27 in. Hg. will be achieved at and during firing or fusing cycle. Through this procedure each firing will produce bridge or crown products having consistently true color, glaze and =,tra-nslucency. It is important that care be taken to assure that the application of vacuum to the products being fired occur before the outer surface of the product becomes sealed'by vitrification. Thus, the product preferably is subjected to. a vacuum atmosphere incident to commencement of the firing of the same.

The vacuum furnace assembly may be seen to best advantage in the schematic showing of FIG. 2 as well as the detail presentation of FIG. 3. The furnace is formed by housing 30 including a top, bottom, side, I

end and front walls 32-42. The housing may be a cast unit or formed by sheet metal sections secured together and otherwise sealed. The enclosure, likewise, may be formed of sheet metal sections.

the operation ofa solenoid valve 74 which is located in the connector 62. Air flow through regulator 76 may be further regulated in pressure by the solenoid valve thereby to regulate the vacuum force. The vacuum may be read on gauge 78.

The vacuum chamber 46 is in communication with a manifold 80 in the vacuum module. This communication is through a connector pipe 82 and elbow connectors 84, 86, one of which is directly coupled to the chamber whereas the other is coupled to the manifold through an elbow 88. Thus, upon the development of vacuum conditions within the transducer, air is drawn from the chamber to the manifold by the described The manifold is in the closed flow path and, therefore, under the same-or approximately the same vac- -uum conditions as is the chamber. Thus, the gauge 78 maynead this vacuum by direct connection'through tube 104 to the manifold. The drawn flow will be unidirectional by interposition of a check valve (not shown l within the housing 92. Operation of the vacuum module iscontrolled by vacuum On-Off, switch-l06. The switch is mounted .on the front panel of the module. The panel also mountsa vacuum relief switch 108. The latter switch controls theopening and closing of an outlet tube connected to the manifold.

The muffle assembly is received within the vacuumchambet (See FIG. 2 Details of the muffle assembly may be seen to best advantage in FIG. 4. As illustrated, the muffle assembly includes a frame or subassembly l 22 having a generally cylindrical wall 124 extending between a front disc .126 and a back plate 128. The front disc may be spot welded or otherwise secured to the cylindrical wall. The front disc 126 may also be spot welded or.otherwise secured to the inner wall of the vacuum chamber. Preferably the muffle assembly will be disposed rearwardly of the chamber although in somewhat close proximity so that the door of the muffle may easily be openedand product introducedand removed. The back plate is received within the confines of the .wall 124 and secured by a plurality of metal screws 130. The front plate is provided with anopening 132 preferably. of rectangular outline.

The muffle 134 provides'a curved top wall throughout but otherwise is of rectangular cross-section. The

The muffle back is formed by a solid body providing a recess in the forward surface for receipt of the throat. Preferably, the muffle will be cemented to the muffle back and the muffle throat. A suitable electrical refractory cement may be used for this purpose.

As illustrated in FIG. 4, the muffle 134 is insulated from the sub-assembly 122. The ceramic fiber in bulk form may be used for this purpose. A door is mounted on the subassembly at the recess 132. The

. door is pivotably mounted for ease in opening, as by a .pair of tweezers.

Referring to FIG. '2, the muffle surface supports a winding 121 throughout its length. The winding is preferably of platinum wire and embedded in the refractory cement. The'winding is connected to a current input at terminal stud 142. The'terminal stud is threadedly received in the back plate and connected to the platinum wire winding b conductors 144, 146, which pass through a pair of openings in the muffle back and back Plate. I

As has been discussed and which will be more particularly described below, the muffle temperature and "temperature rise "during a firing cycle is controlled. A Qs'ensing device such as a platinum, 13% rhodium thermo'couple may be used as a part of the control device. The thermocouple is passed through an opening 152 in the back plate and muffle back for receipt within the confinesof the muffle. The particular temperature program, i.e. the incremental increase of 86F. per

minute from an initial temperature of 1400F. over a period of five minutes is controlled through the firing cycle of the apparatus, including the current controller 16 and the automatic temperature controller 18.

The programmed temperature cycle may be explained with reference to FIG. 7 representing the schematically operative components and the circuitry of operation. The operative componentsinclude a linear temperature programmer assembly 160, a relay assembly 180, a controller assembly 200, and a variable time proportioning switch assembly 220.

Any straight linetemperature firing cycle, within +5F., may be attained by first setting the time proportioning switch 220 to a desired rate of climb, thereafter setting the lever 162 and scale 164 to the desired temperature, and finally startingthe program through On-Off switch 166. The switch 166 energizes relay 180 thereby to carry out the various operative functionsof the system through various relay switches (not shown).

' At the commencement of the cycle of operation, the relay 180 opens the circuit of potentiometer 202 of controller 200 and closes the circuit of potentiometer 168 of the programmer forming a portion of the circuit of controller 200.- Relay operation causes switches in the circuit of motor 170 and proportioning control 220 to close. The proportioning control determines the On-Off" time during which the motor drives potentiometer 168 and scale 164. As the electrical resistance of the potentiometer 168 changes the controller 200, if the actual temperature sensed by the thermocouple 150 is below the theoretical temperature to be sensed, current input will be required. The controller serves to satisfy the current demand in attaining the theoretical or programmed temperature cycle within a factor of +5F. of temperature on the scale 164.

When the temperature as set on the scale 164 is reached, the lever 162 functioning as a switch arm causes the microswitch 172 to open. The relay deenergizes and the reverse of the operation discussed ensues. Thus, the circuit potentiometer 202 closes to return the temperature control to the controller 200 and the motor 170 and the time proportioning controller become dc-energized. An alarm which may either be visual or audible, or both, may be energized to provide an indication of the completion of a firing cycle.

From the foregoing, it is seen that, in accordance with the present invention, there is provided a novel control to program the temperature of a furnace chamber through the firing cycle and to carry out this control with a furnace chamber subjected to a constant negative pressure at commencement of and during the firing cycle. 7 1

Having described the invention with particular reference' to the preferred form thereof, it will be obvious to those skilled in the art to which the invention pertains.

after understanding the same, that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined 'by the claims appended hereto.

We claim:

1. An electrically heated furnace for use in firing dental products comprising a housing and a chamber therein which is sealed from exterior ambient conditions. a muffle, means for supporting said muffle within said housing, said muffle including a walled enclosure providing an interior compartment adapted to be heated, electrical resistance means supported by the walls of said enclosure and extending along the length of said compartment, and a door, said door mounted by said enclosure to permit access to said compartment,

vacuum generating means, means connecting said vacuum means to said chamber so that the pressure therein may be reduced to a prescribed value, electrical means for programming linearly over a predetermined time period the temperature rise in said muffle compartment to a maximum during a firing cycle, power means, said programming means including circuit means connected to said resistance means, means for controlling input of said power means to said resistance means to provide linear rise in temperature over said'firing cycle, a thermocouple, said thermocouple supported within and responsive ,to the temperature of said compartment, said thermocouple electrically connected to said circuit means, said control means responsive to temperature 7 response to said thermocouple, and a door providing access to said chamber to remove fired products, said door being supported by said enclosure.

2. The furnace of claim l wherein said programming means includes means for maintaining the; temperature of said muffle compartment at a minimum set temperature, means for setting said maximum temperature for said muffle, and time proportioning control means for determining a temperature rate of'climb over a cycle of operation. 7

3. The furnace of claim 2'including relay means, control switch means energizing said relay-means for commencing said cycle of operation, drive means, variable resistance means, said drive means energized by said relay means for driving said variable resistance means to change said internal resistance, said control means responsive to said resistance change to provide power input. 9 I

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,909,590 DATED September 30, 1975 l-NV.ENTOR 5) 3 Robert A. Jensen, John A. Tesk and Daniel Odulio It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 33, "The throat of the 8 should read --The throat of the venturi is in communication with the vacuum port 58.

Sr'gncd and Scaled this twenty-fourth Day Of February 1976 [SEAL] A nest:

RUTH c. MASON c. MARSHALL DANN Arresting Officer Commissioner nj'larenls and Trademdrks UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 8,909,590

DATED September 30, 1975 |NVENTOR(5) 3 Robert A. Jensen, John A. Tesk and Daniel Odulio It is certified that error appears in the ab0veidentified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 33, The throat of the 8 should read --The throat of the venturi is in communication with the vacuum port 58.

Signed and Scaled this twenty-fourth Day Of February 1976 [SEAL] AfleSI.

RUTH c. MASON c. MARSHALL DANN Arresting Officer (ommissimur uflarenls and Trademizrks 

1. An electrically heated furnace for use in firing dental products comprising a housing and a chamber therein which is sealed from exterior ambient conditions, a muffle, means for supporting said muffle within said housing, said muffle including a walled enclosure providing an interior compartment adapted to be heated, electrical resistance means supported by the walls of said enclosure and extending along the length of said compartment, and a door, said door mounted by said enclosure to permit access to said compartment, vacuum generating means, means connecting said vacuum means to said chamber so that the pressure therein may be reduced to a prescribed value, electrical means for programming linearly over a predetermined time period the temperature rise in said muffle compartment to a maximum during a firing cycle, power means, said programming means including circuit means connected to said resistance means, means for controlling input of said power means to said resistance means to provide linear rise in temperature over said firing cycle, a thermocouple, said thermocouple supported within and responsive to the temperature of said compartment, said thermocouple electrically connected to said circuit means, said control means responsive to temperature response to said thermocouple, and a door providing access to said chamber to remove fired products, said door being supported by said enclosure.
 2. The furnace of claim 1 wherein said programming means includes means for maintaining the temperature of said muffle compartment at a minimum set temperature, means for setting said maximum temperature for said muffle, and time proportioning control means for determining a temperature rate of climb over a cycle of operation.
 3. The furnace of claim 2 including relay means, control switch means energizing said relay means for commencing said cycle of operation, drive means, variable resistance means, said drive means energized by said relay means for driving said variable resistance means to change said internal resistance, said control means responsive to said resistance change to provide power input. 